Science and Physiology Help

gotgoo

physio_1_lab_6.pdf

Home >Biology homework help >Science and Physiology Help

Lab 6. The Nervous System – Somatic &

Special Senses

LEARNING OBJECTIVES

Instructions: When you have mastered the objective, place a check mark in the box.

At the completion of this exercise, you will be able to:

1. Analyze the physiology of somatic senses by conducting experiments to determine the

relative touch acuity of various regions of the hand. ☐ 2. Analyze the physiology of somatic senses by conducting an experiment to demonstrate

touch receptor adaptation. ☐ 3. Analyze the physiology of somatic senses by conducting an experiment to simulate

referred pain. ☐ 4. Analyze olfactory receptor physiology by conducting an experiment to observe olfactory

receptor adaptation. ☐ 5. Analyze taste receptor physiology by conducting an experiment to define the sites of taste

receptor modalities. ☐ 6. Evaluate visual systems by conducting tests to determine visual acuity, astigmatism,

near-point of accommodation, red-green color blindness, position of the blind spot. ☐

7. Analyze the auditory system by conducting a test to define areas of sound localization. ☐

INTRODUCTION

The ability of the nervous

system to respond to constantly

changing environmental

conditions depends first on the

ability of the nervous system to

detect environmental changes,

called stimuli. Stimuli are

detected by a wide variety of

receptors located inside and

outside the body. Receptors

function to convert stimuli into

the electrical language of the

nervous system, the action

potential. Receptors may be found in

either at nerve endings or on

specialized cells, called receptor cells. In both cases, receptors convert stimuli into electrochemical

signals that are transmitted to the central nervous system, by graded potentials and action potentials,

along afferent sensory neurons. This information travels along a fixed pathway through the nervous

system.

Once in the nervous system, sensory information must be sent to a specific region of the brain for

processing. The result of processing is awareness, also called perception. Many different types of

Receptor Adequate Stimulus

Photoreceptors Light

Chemoreceptors Molecules/ions (Ex. O2, CO2 H

etc…)

Baroreceptors Pressure

Touch receptors Distortion of skin

Proprioceptors Limb/body position

Nociceptors Tissue damage (perception of pain)

Thermoreceptors Heat/cold

Osmoreceptors Body fluid osmolarity

Stamp for

Credit

Table 1. Adequate stimuli of various sensory receptors.

stimuli may elicit receptor activation. However, each receptor type has an optimal stimulus that activates

it most efficiently, called the adequate stimulus. Table 1 shows a list of receptor types and their

adequate stimuli. Awareness of a stimulus is due to processing in the central nervous system, not at the

receptor. Therefore, what is perceived depends on the receptor that was activated, not on the stimulus

that activated it. For example, the adequate stimulus of photoreceptors is light. Mechanical stimuli, e.g.,

rubbing the eyes also results in the perception of flashes of light. Since the sensory information travels

on a fixed pathway, whatever activates a photoreceptor will end up sending information to the region of

the brain that processes light. Although photons of light were not the stimuli, the perception of

mechanical stimulus on the photoreceptors, will be flashes of light. This phenomenon is an example of

The Law of Specific Nerve Energies, whereby the nature of perception is defined by the pathway over

which the sensory information is carried.

Receptor adaptation is a common property of all types of receptors and is particularly relevant

to your study of sensory receptors. Many receptors have the ability to stop or simply reduce their

response to a constant stimulus. For example, when initially stepping into a hot bath, a person may feel

that the water is extremely hot. After awhile, the water will feel comfortable. Receptors that adapt to

stimuli are called phasic receptors. Tonic receptors on the other hand rarely stop responding to a

constant stimulus.

In this lab, you will explore the physiological effects of various stimuli on the human body.

TERMS TO DEFINE BEFORE LAB

1. Perception: ____________________________________________________________________ _________________________________________________________________________________

_________________________________________________________________________________

2. Adequate stimulus: ______________________________________________________________ _________________________________________________________________________________

_________________________________________________________________________________

3. The Law of Specific Nerve Energies: _______________________________________________ _________________________________________________________________________________

_________________________________________________________________________________

4. Receptor adaptation: ____________________________________________________________ _________________________________________________________________________________

_________________________________________________________________________________

5. Phasic receptor: ________________________________________________________________ _________________________________________________________________________________

_________________________________________________________________________________

6. Tonic receptor: ________________________________________________________________ _________________________________________________________________________________

_________________________________________________________________________________

LABORATORY PROCEDURES

 In Activity 1, you will conduct various experiments to analyze the general somatic senses.

 In Activity 2, you will conduct various experiments to analyze chemoreceptor function.

 In Activity 3, you will conduct various experiments to evaluate visual senses.

 In Activity 4, you will conduct an experiment to determine auditory acuity.

ACTIVITY 1 – ANALYZE THE GENERAL SOMATIC SENSES

The somatic senses are derived from receptors in the body wall, such as those that sense touch,

temperature, pain, joint proprioception, etc. The following laboratory experiments focus on touch and

pain receptors. Both touch and pain receptors monitor an area called the receptive field, which is a

region of space that will alter firing of a sensory neuron. A touch receptor on the general body surface,

for example, may be stimulated if a touch stimulus occurs within its receptive field of about 2.5 inches

in diameter1. Receptive field size and receptor density contribute to touch acuity. Touch acuity is a

measure of how precisely the location of a stimulus is detected. In areas of the skin where acuity is low,

the receptive field tends to have a larger diameter and receptors occur in lower densities. Areas with

high acuity are composed of receptors with a receptive field having small diameter, and these receptors

tend to occur in high densities.

When the receptive fields of different sensory receptors overlap, sensory inhibition may occur.

This often occurs in high acuity locations, such as the hands. Sensory inhibition may also occur when

the high magnitude of stimulus at one receptor masks the lower magnitude stimulus of an adjacent

receptor. Inhibition of sensory input due to overlapping receptive fields or magnitude of stimuli, are

examples of lateral inhibition. We will use two tests to determine touch acuity: (1) the touch

localization test, and (2) the two-point discrimination test.

Pain is also a somatic sense. Pain receptors are found on free nerve endings and are generally

sensitive to many different types of stimuli. These stimuli are associated with tissue damage, e.g., the

concentration of extracellular K+, extreme temperature changes, extreme pH changes, etc. A common

phenomenon associated with nociception (pain), is referred pain. Referred pain is the perception of

pain in a region of the body that does not contain tissue damage. Instead, the pain is due to tissue

damage in another region of the body, often an internal organ. This phenomenon occurs, because pain

receptors from internal organs may send pain information along the same neural pathway in the CNS as

pain neurons from regions of the skin. Thus, when this information reaches the brain, the brain processes

the sensory input and perceives that the pain originates at the surface of the skin, instead of at an internal

organ. An excellent example of referred pain is the radiating pain in the left arm and shoulder during a

heart attack.

Experiment 1 – Conduct an experiment to determine sensory acuity of touch receptors using the touch-

localization test.

1. Obtain two pens, one for you and the other for your lab partner.

2. Ask your lab partner to close their eyes and keep them closed until all steps are completed

3. Use your pen to touch a random spot on your partner’s palm (not too hard).

4. Remove your pen. A mark should be left in the spot in which it was originally placed.

5. Ask your partner to touch the exact spot you touched. Your partner should use his/her pen to

mark the spot while keeping his/her eyes closed.

6. Measure the distance in mm between your pen mark and your partner’s pen mark.

7. Complete the experiment a total of three times, choosing a slightly different spot on the palm.

Record the results in the data table below.

a. Calculate an average distance for the three trials

8. Repeat these procedures for the following regions:

a. Back of the hand (dorsal surface)

b. Fingertip

c. Upper back

d. Ventral upper arm

Region Tested Trial 1

Distance (mm)

Trial 2 Distance

(mm)

Trial 3

Distance (mm)

Average Distance

(mm)

Palm of Hand

Back of Hand

Fingertip

Upper back

Ventral upper

arm

Instructions: Answer the questions below in complete sentences.

1. Compare and contrast the distances measured between the highest average distance and lowest average distance. Which indicates better sensory acuity? Explain why. Use the words: receptive

field, touch receptor, and acuity. Under line or highlight these words in your explanation.

2. According to the data, which regions of the hand have the lowest acuity? Explain why you think this occurred. Use the words: receptive field, average distance, touch receptor, and acuity.

Underline or highlight these words in your explanation.

3. According to the data, which regions of the hand have the highest acuity? Explain why you think this occurred. Use the words: receptive field, average distance, touch receptor, and acuity.

Underline these words in your explanation.

Experiment 2 – Conduct an experiment to determine sensory acuity using the two-point discrimination

test.

1. Obtain a paperclip and a small metric ruler

2. Open up the paperclip into a “U” shape, so that you may control the distance between the two

points at the ends of the paperclip.

3. Your lab partner must close their eyes for the remainder of the experiment.

4. By trial and error, you must determine the furthest distance between the two points of the

paperclip that can be distinguished as two points, by your lab partner, in that region.

a. Start with the points as close together as you can get them and gradually increase the

distance between the two points.

b. Try to keep your partner guessing by randomly touching with one point and both points

to avoid error from anticipation.

5. Measure the distance in three trials with a metric ruler and report the distances in the data table

below:

Region Tested Trial 1

Distance (mm)

Trial 2

Distance (mm)

Trial 3

Distance (mm)

Average Distance

(mm)

Ventral Thumb

Palm of Hand

Back of Hand

Ventral

Forearm

Ventral upper

arm

Upper back

Instructions: Answer the following questions using the data above to justify your claims.

1. Compare and contrast the distances measured between the highest average distance and lowest average distance. Which indicates better sensory acuity? Explain why. Use the words: receptive

field, touch receptor, and acuity. Under line these words in your explanation.

______________________________________________________________________________

2. According to the data, which regions of the hand have the lowest acuity? Explain why you think this occurred. Use the words: receptive field, average distance, touch receptor, and acuity.

Underline or highlight these words in your explanation.

______________________________________________________________________________

3. According to the data, which regions of the hand have the highest acuity? Explain why you think this occurred. Use the words: receptive field, average distance, touch receptor, and acuity.

Underline or highlight these words in your explanation.

______________________________________________________________________________

Experiment 3 – Conduct an experiment to demonstrate touch receptor adaptation.

1. Obtain three coins of the same size (pennies) for you and your partner to use in this experiment.

2. Rub one penny between your palms for a few seconds to bring it to your external body

temperature.

3. Place one penny on the ventral surface of your forearm.

4. Record the time (in seconds) until the touch sensation disappears: _______________________.

5. Repeat the experiment with three coins on a different section of your forearm.

6. Record the time (in seconds) until the touch sensation disappears: _______________________.

Instructions: Answer the following questions using data to justify your claims.

1. Are your results indicative of a tonic or phasic receptor? What specifically led you to this conclusion?

______________________________________________________________________________

2. Why is touch receptor adaptation beneficial when you wear clothes? ______________________________________________________________________________

______________________________________________________________________________

Experiment 4 – Conduct an experiment to simulate referred pain.

1. Dip your elbow in a bath of ice and water deep enough to cover your entire elbow.

2. Keep your elbow submerged until you feel a tingling in your fingertips.

3. Keep track of the time it takes to feel the sensation: _____________________.

Instructions: Answer the questions below.

1. In your own words, explain why you experience pain in a region of the body that has no applied stimulus.

______________________________________________________________________________

ACTIVITY 2 – ANALYZE CHEMORECEPTOR FUNCTION

Sensory receptors in the body may be organized by their position inside the body

(interoreceptors) or outside the body (exteroreceptors). Exteroreceptors are localized in a position that

is key to relaying information from the external environment to the CNS, while interoreceptors help the

body to maintain internal homeostasis. Here, we consider two types of exteroreceptors, which are also

classified as chemoreceptors: (1) olfactory receptors and (2) taste receptors.

Chemoreceptors provide information about molecules dissolved in saliva and in the air.

Odorants are molecules in air that bind to olfactory receptors on the olfactory epithelium in the nasal

cavity. Olfactory receptors bind a diverse array of odorant molecules. In the olfactory epithelium there

are about 35 different types of olfactory receptors, yet these receptors can help you distinguish 2000-

4000 different chemical stimuli. On the apical surface of olfactory receptor cells, cilia extend into the

mucus covering the olfactory epithelium. This mucus traps and concentrates odorant molecules,

allowing detection of odorants present in very low concentrations.

Olfactory receptors are a classic example of phasic receptors, discussed earlier. For example, if

you are in a classroom with a fellow student wearing too much cologne or perfume, the smell is initially

very intense. However, over time the smell is not as intense, because your olfactory receptors slow or

stop responding to the odorant.

Taste receptors are concentrated on the surface of the tongue, but are also sparsely located

throughout the oral cavity. Close observation of the tongue will reveal that there are many small bumps

on its surface. These bumps are called papillae. The three basic types of taste papillae are: (1)

fungiform papillae, containing a small number of taste buds; (2) circumvallate papillae, which contain

a large number of taste buds; and (3) filiform papillae, which contain no taste buds or taste receptors,

but rather a small hair not involved in gustation.

Taste buds in fungiform and circumvallate papillae contain taste receptor cells and stem cells

that constantly replace old taste receptor cells. There are five types of taste receptor cells that sense each

of the following: (1) sweet, (2) bitter, (3) salty, (4) sour, and (5) umami. There are only 5 taste receptor

types, and thus, only 5 taste modalities. All other flavors are either a combination of taste and smell or

just smell. Table 2 shows the five taste receptor types and examples of adequate stimuli. All regions of

the tongue are sensitive to all taste stimuli with one exception, the center of the tongue, which contains

only filiform papillae and is taste-blind. Although as you will see different regions of the tongue are

more sensitive to certain taste modalities.

In the experiments below, you will look for differences in sensitivity in different regions of the

tongue when stimulated with, sweet, sour, bitter, and salty taste stimuli.

Experiment 5 – Conduct an experiment to demonstrate olfactory receptor adaptation.

1. Use your finger to hold one nostril closed.

2. Open the bottle of Oil of Wintergreen or other “smelling solution” provided.

3. Use your free hand to wave odors from the opening of the bottle towards your open nostril.

4. Record the time in seconds for the odor to disappear: ____________________

Experiment 6 – Conduct an experiment to localize various taste receptor modalities.

1. You and your partner must move to the tasting solutions station in the lab. Do not move the

solutions.

2. Have your partner blot their tongue dry with a clean paper towel.

3. Without showing your partner the identity of the solution, dab a cotton swab soaked in the

glucose solution onto the tip of their tongue. Record a + if they can taste it or a – if they cannot

taste it in the table below.

4. Have your partner blot their tongue dry with a clean paper towel.

5. Without showing your partner the identity of the solution, dab a cotton swab soaked in the

vinegar solution on the tip of their tongue. Record a + if they can taste it or a – if they cannot

taste it in the table below.

6. Have your partner blot their tongue dry with a clean paper towel.

7. Without showing your partner the identity of the solution, dab a cotton swab soaked in the

sodium chloride (NaCl) solution on the tip of their tongue. Sodium chloride is table salt, so it is

edible. Record a + if they can taste it or a – if they cannot taste it in the table below.

8. Have your partner blot their tongue dry with a clean paper towel.

9. Without showing your partner the identity of the solution, dab a cotton swab soaked in the

quinine solution (or tonic water) on the tip of their tongue. Record a + if they can taste it or a – if

they cannot taste it in the table below.

10. Repeat the steps above for the sides of the tongue and the back of the tongue

a. Use new cotton swabs each time, DO NOT DOUBLE DIP!

Tongue

Location

Salt

(NaCl)

Sour

(Vinegar)

Bitter

(Quinine)

Sweet

(Glucose)

Tip

Sides

Back

Instructions: Answer the questions below.

1. Why did we not take taste measurements from the center of the tongue? Explain. ______________________________________________________________________________

______________________________________________________________________________

2. While drinking hot soup, you burn the tip of your tongue. Explain what papillae are affected and which taste modality is lost. Refer to your data in your explanation.

______________________________________________________________________________

3. While chewing on a large piece of steak, you accidently bite your tongue on the right side only. Will this affect your taste? If so, which taste modality is affected. Which papillae are affected?

______________________________________________________________________________

ACTIVITY 3 – ANALYZE THE VISUAL SENSES

Light is one of many forms of electromagnetic radiation that travel in waves. The properties of

this form of energy are determined by the wavelength, or the distance between a point in adjacent

waves, and by amplitude (size) of the waves. For example, different colors of visible light are

determined by the wavelength of electromagnetic radiation.

Light entering the eye is diffracted (or bent) as it enters the transparent cornea. The amount of

diffracted light that enters the eye is regulated by the iris. Light that passes the iris is again diffracted by

the lens. Light passing through the lens is bent so that the rays of light are focused onto the retina in the

back of the eye. The retina contains the photoreceptor cells called rods and cones that convert the

light/image into electrical signals that are processed by the brain. Rods are the most common type of

photoreceptor cell throughout the retina, except at the macula lutea.

Rods are photoreceptors that are sensitive to a wide range of light wavelengths, though are not

used for color vision. Rods contain a photopigment called rhodopsin, which is a combination of an

opsin protein and a light sensitive molecule called retinal (synthesized from vitamin A). Rods are used

in situations where light is dim, and these photoreceptors are well suited for detection of movement.

The macula lutea is a region of the retina where cones are concentrated, and are highly

concentrated at the center of the macula lutea, a region called the fovea centralis. Cones require lots of

light to function properly, but allow for color vision and provide the sharpest vision. Three different

types of cones sense light in the red, green, and blue regions of the electromagnetic spectrum. All three

types of cones contain retinal but differ in the opsin protein they use to produce the photopigment. The

gene for the red cone opsin and the green cone opsin are found on the X-chromosome, and if defective,

causes red-green colorblindness. Unlike females, males have only one X-chromosome, making them

more susceptible to developing red-green colorblindness.

The retina is a neural tissue composed of neurons and glial cells. The neurons that carry

information, from the photoreceptors out of the eye, do so using axons. In one section of the retina,

known as the optic disc, these axons leave the retina and enter the optic nerve. At this site there are no

photoreceptor cells at all. If an image is projected onto the optic disc the object cannot be seen, since

there are no photoreceptors to detect the image. For this reason, the optic disc is commonly known as the

“blind spot”.

Ensuring that the image is projected directly onto the retina, and more specifically on the fovea

centralis region of the retina is essential to our ability to see objects clearly. If the lens were a fixed lens,

meaning it could not move or change shape, focusing would only be possible if you stood at a fixed

distance from an object you wanted to see clearly. If you moved forward that object would be focused

behind the retina (hyperopia or far-sightedness); if you moved backwards the object would be focused

in front of the retina (myopia or near-sightedness). Either condition results in perception of a blurry

image. Luckily, our lens is capable of changing shape, allowing us to focus light directly onto the retina.

When the lens changes shape to project an image onto the retina this is called accommodation.

To project an image that is far away from your eye, ciliary muscles around the lens relax pulling it into a

flattened shape. To focus the image of a nearby object onto the retina, the ciliary muscles relax, reducing

reduces tension on the lens; due to the elasticity of the lens, it recoils to a round shape. The ability of the

lens to recoil, allowing us to see nearby objects, is an ability we slowly lose with age. In other words, if

you live long enough, you will eventually require reading glasses to see words on a page. The technical

term for the far-sightedness (unable to see objects nearby), is presbyopia.

Visual acuity is a measure of the sensitivity of your visual system and is usually measured using

a Snellen eye chart. The Snellen eye chart compares a person’s visual acuity with that of an individual

with normal vision. When reading a line on the eye chart, you will notice a fraction next to the line you

just read. The numerator indicates the distance (feet) at which you are reading the line and the distance

at which a person with normal vision can read that line. For example, if the line you fail to read contains

the fraction 20/50. This means you have worse than normal vision, because you cannot read a line at 20

feet that a person with normal vision can read at 50 feet. Decreasing visual acuity has many causes, one

of which is known as an astigmatism.

Astigmatism is a loss in visual acuity due to a misshapen eye ball. Because the eye ball is

misshapen, the distance between the lens and the retina is abnormal. Subsequently, the lens projects the

image in front or behind the retina, leading to a loss of visual acuity. Minor astigmatisms are common. If

the astigmatism is minor, a person may go for many years with the condition undiagnosed.

Experiment 7 – Conduct an experiment to evaluate your visual acuity using the Snellen eye chart.

1. You and your lab partner must go over to the Snellen eye chart.

2. The person testing their visual acuity should stand 20 feet from the chart and the other person

should stand next to the chart.

3. The person doing the test should do both a corrected (with glasses) and uncorrected (without

glasses) version of the test if possible

 People with contacts only need to do the corrected version.

4. To begin the test cover your right eye and read the smallest line of letters you can out loud to

your partner.

5. Your partner should note the rating (d/D fraction) located next to the line your partner fails to

read correctly in the table below.

6. Repeat the steps above covering the left eye, and again with none of your eyes covered.

d/D Uncorrected d/D Corrected

Left eye

Right eye

Both eyes

Instructions: Answer the following questions, using data to justify your claims.

1. What is considered normal visual acuity (uncorrected)? ______________________________________________________________________________

______________________________________________________________________________

2. Would someone with 20/15 vision have normal vision? Is this better, worse or normal? Explain. ______________________________________________________________________________

______________________________________________________________________________

3. Explain the difference between myopia and hyperopia? ______________________________________________________________________________

______________________________________________________________________________

4. Explain how do glasses correct blurred vision (myopia and hyperopia)?

______________________________________________________________________________

5. How do contact lenses correct blurred vision? ______________________________________________________________________________

______________________________________________________________________________

Experiment 8 – Conduct a test to evaluate your vision for astigmatism.

1. You and your lab partner must go over to one of the astigmatism charts.

2. The person testing for astigmatism must stand 10 feet from the chart, and the chart must be well

lit.

3. Begin by looking at the chart with the left eye covered.

4. If you see any lines on the chart as blurry or if some lines appear darker than others you have

astigmatism.

5. Repeat the steps above with right eye covered.

Instructions: Answer the following questions.

1. What is astigmatism and how does it affect normal vision? Explain in your own words. ______________________________________________________________________________

______________________________________________________________________________

2. How is astigmatism corrected? ______________________________________________________________________________

______________________________________________________________________________

Experiment 9 – Conduct an experiment to evaluate presbyopia using the near point of accommodation

test.

1. Obtain a meter stick and a pencil with a sharpened tip.

2. Cover the left eye and hold the pencil away from your right eye at arm’s length with the pencil

tip pointing upwards. Focus only on the tip of the pencil.

3. Slowly move the pencil closer to your right eye. When the tip of the pencil appears to double or

becomes blurry stop moving it, and measure the distance from your right eye with the meter

stick.

 Near point distance from the right eye: _________________________

4. Repeat the steps above with your left eye and the right eye covered.

 Near point distance from the left eye: _________________________

Instructions: Answer the following questions, referring to data where appropriate.

1. Using the table below estimate the age of your right and left eyes. The y-axis measures

diopters measured in reciprocal meters (1/meters).

a. Right eye: _________________

b. Left eye: __________________

2. What is presbyopia? ___________________________________________________________________________

___________________________________________________________________________

3. How is presbyopia corrected? ___________________________________________________________________________

___________________________________________________________________________

Experiment 10 – Conduct an experiment to evaluate whether you have red-green colorblindness.

1. You and your partner must use a laptop computer from the cart. Plug in the laptop computer, and

start it up.

2. Open up Internet Explorer, and type the following into the address bar:

http://www.colour-blindness.com/colour-blindness-tests/ishihara-colour-test-plates/

3. Read the instructions at the top of the webpage carefully. As you go through all 24 plates, answer

the questions from select plates below.

Instructions: Answer the following questions.

1. How many of the numbers indicating colorblindness did you see in plates 1-7? ______________________________________________________________________________

____________________________________________________________________________

2. How many of plates 8-13 seemed to be missing a visible number in the plate? ______________________________________________________________________________

______________________________________________________________________________

3. If 1 or more were missing a number, what does this indicate? ______________________________________________________________________________

______________________________________________________________________________

4. Did you see a number for plates 14 or 15? ______________________________________________________________________________

______________________________________________________________________________

5. If you did not, what does this indicate? Explain. ______________________________________________________________________________

______________________________________________________________________________

6. What is protanopia? ______________________________________________________________________________

______________________________________________________________________________

7. If a person has protanopia what should they see in plate 18? Why or why not? ______________________________________________________________________________

______________________________________________________________________________

8. What is deuteranopia? ______________________________________________________________________________

______________________________________________________________________________

9. If a person has deuteranopia what should they see in plate 18? Explain. ______________________________________________________________________________

______________________________________________________________________________

http://www.colour-blindness.com/colour-blindness-tests/ishihara-colour-test-plates/

Experiment 11 – Conduct an experiment to determining the position of your blind spot.

1. Obtain a card with a circle and a cross.

2. Hold the card about 20 inches from your right eye with your left eye covered. If a card is

unavailable, access an online version of this exercise at the following URL:

http://serendip.brynmawr.edu/bb/blindspot1.html.

a. Be sure that the circle is in line with your right eye and the cross is off to the right side.

3. With your right eye, stare at the cross. You will see the circle, but don’t stare at it. Just notice it’s

there. If you cannot see the circle, move the card back farther until you do see the circle. While

staring at the cross, slowly move the card closer until the cross disappears.

4. Continue to move the card closer to your eye and you will see the cross reappear.

5. Repeat the above steps with the right eye covered and the left eye focused on the circle.

Instructions: Answer the following questions.

1. What structure in the eye is the cross focused on when it disappears?

______________________________________________________________________________

2. Explain why are you unable to see the cross when it is focused on this structure?

______________________________________________________________________________

ACTIVITY 4 – ANALYZE THE AUDITORY SENSORY MODALITY

Sound, like light, also travels in waves. Sound waves are initiated by vibrations that are

transferred from an object to the surrounding medium and radiates in all directions. Structures in the

inner ear transform the vibrations into nerve impulses, which are interpreted by the brain as sound.

Structures in three regions of the ear contribute to gathering vibrations, transferring vibrations, and

transforming vibrations into neural impulses: (1) the outer ear, (2) the middle ear, and (3) the inner ear.

The outer ear collects sound waves, which are converted into mechanical vibrations by the tympanic

membrane (ear drum). The auditory ossicles in the middle ear transfer the vibrations to the inner ear by

causing vibrations in the perilymph of the scala vestibuli of the cochlea. Vibrations of the perilymph are

transferred to a membrane in the cochlea, called the tectorial membrane, which rubs against hair cells in

the organ of Corti. The inner and outer hair cells transform the vibrations into nerve impulses that are

sent to the brain via the cochlear branch of the vestibulocochlear nerve (CN VIII). These structures

allow humans to hear various frequencies of sound and allow us to determine the position of sounds.

To localize sound, the brain analyzes the difference in volume in both ears as well as how

quickly the sound reaches each ear. If the source of a sound is closer to the right ear the sound will arrive

at the right ear first and be slightly louder in the right ear. The areas where these differences are reduced

are areas where sound localization becomes difficult such as any sound close to midline. If a sound is

produced behind your head, around the midline, for example, the sound will arrive in both ears at the

http://serendip.brynmawr.edu/bb/blindspot1.html

same time. If you do not have any form of hearing loss, you will hear the same volume in both ears. In

the following experiment, you will conduct an experiment to demonstrate sound localization.

Experiment 12 – Conduct an experiment to demonstrate sound localization.

1. While your lab partner has his/her eyes closed strike a tuning fork on a rubber stopper on the

desktop behind your lab partner.

2. Randomly select a position to place the vibrating tuning fork around your partner’s head. Then

have your partner point in the direction they hear the sound coming from. Do at least 3 trials with

at least one of them directly behind your lab partner’s head.

Instructions: Answer the following question.

1. In which position did your partner have the most difficult time pointing to the location of the

sound from the tuning fork? Explain why?

______________________________________________________________________________

2. Create a diagram showing how sound is created by an object, transmitted through the structures

of the ear, and transformed into neural impulses by structures in the inner ear. Label all of the

following structures in your diagram: organ of Corti, inner hair cell, outer hair cell, tectorial

membrane, perilymph, inner ear, middle ear, outer ear, external acoustic meatus, external

auditory canal, tympanic membrane, stapes, incus, malleus, oval window, and cochlea.

Diagram: